The present invention relates to an internal thread member, an external thread member, and a cutter for machining an internal thread.
As basic screw shapes, the JIS standards have included descriptions of, for example, miniature thread screws, pipe screw threads, metric screw threads, unified screw threads, and cycle threads. Specifically, please refer to JIS B 0201 “Miniature Screw Threads,” JIS B 0202 “Parallel Pipe Threads,” JIS B 0205 “Metric Coarse Screw Threads,” JIS B 0206 “Unified Coarse Screw Threads,” JIS B 0216 “Metric Trapezoidal Screw Threads,” and JIS B 0225 “Cycle Threads.”
Since threads are cut in an external thread along its outside surface, it is easy to machine the external thread. According to the JIS standards, for performance reasons, a rounded root of the external thread is designed to have a large radius, and its height of thread engagement on the external thread side with reference to its pitch diameter that constitutes a part of its thread overlap is smaller than its height of thread engagement on the internal thread side that constitutes the remaining part of the thread overlap.
Concerning metric screw threads and unified screw threads, no specific requirements are established with regard to the rounded roots of their internal threads. However, the height from the top of a fundamental triangle to the top of an external thread root is H/8 (H=(√3/2) P; where P represents the pitch of the thread). Accordingly, when the pitch is 1 mm, the radius of a rounded root is approximately 0.072 mm, which means that the rounded root is very small. However, cycle threads with rounded roots have the largest radius among various types of screws. In the case of the cycle thread, the radius of the rounded root of an internal thread is one-sixth (⅙) of the pitch. The size of the cycle thread is normally expressed in inches. However, assuming that its pitch is 1 mm, the radius of its rounded root would be approximately 0.16667 mm. Therefore, the thread overlap of the cycle thread would be small.
Cutters such as screw taps, end mill taps (or planet taps), and cutting tools are used to grind and machine internal threads. When machining an internal thread that has a rounded root of a small radius, the tip (or crest) of the cutting edge of the cutter tends to wear out quickly and to easily become chipped or to suffer damage. This tendency is particularly strong when the material for the internal thread is hard. Accordingly, there is a possibility that it may become difficult to secure stable finishing accuracy and the life of the cutter may be shortened.
Moreover, as described above, if the rounded root of the internal thread is designed to have a large radius, its thread overlap becomes small, thereby making it difficult to secure sufficient breaking strength and pullout strength.